Puppy biting older dog

We recently welcomed "lulu" into our pack. She was a rescue pup through Poundpaws from a pig dog litter that was dumped.
She's a cool wee character, was bottle fed from 5 days old so she loves people.
She's full of beans and old mate who dumped them would probably be kicking himself if he saw her now and the instinct she's showing.
Problem is, said instinct involves her getting along side Bo and latching onto his face/neck/behind the ears.
She's only playing but Bo is so patient and laid back and her little puppy teeth are like needles so she's caused him quite a lot of damage, drawing blood and giving him little injuries all over his face and neck. He just takes it and won't tell her off and it's starting to do our heads in. We constantly tell her no when we see her doing it and she's had a few whacks on the nose when we've been able to catch her in the act. Trouble is, she's also super smart so instead of stopping it, she's resorted to doing it only when we aren't looking.
She's too small to get a garmin on and I am desperate to knock this behavior on the head as our big beautiful boy is starting to look like he's got a serious Fentanyl problem or something.
Any suggestions?
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Can't help with the wee nipper, but Bo is a very handsome boy.

Put the collar around its stomach area?

This often happens when pups are weaned to early. Mum never had a chance to teach them some manners. When pups play fight an one gets carried away mum will step in and sort it out. Your right to try and stop it because she will continue to do it with others in her "pack" ...including children!. If you know anyone with on older bitch who could visit for an afternoon and "educate" her a little that may help. Sounds harsh but the sooner you knock that kind of behavior it on the head the better. Next time you catch her try holding her down and get Boe to lay on her to establish dominance like her mother would. Don't let her up till she stops wriggling and submits.

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Yup mum would have knocked this on the head straight away

Take the dominant role with the pup. Don’t leave it to Bo. I would begin with dominance exercises - lay the pup on his back and keep him still with your hands. I use the command “ahh” every time the pup moves and clamp him with my hands. Eventually you should be able to put the pup on its back or side and it won’t move until you use a release command. An “Ahh” should stop him if he moves. Next comes the “leave it” command which I’m sure you have on Bo. Use the nipping to introduce the “leave it” command. A little clip on the nose with the leave it might help with the reinforcement.

Had the problem a few years ago with a lab pup, mother was old and a mistake mating. Mother gave up on them at 2 weeks so raised by the owner. Were unaware of this when pup was purchased. At six months pup stated it's stand-over tactics on every dog it came in contact with By 12months no improvement, had been through the exercise of dominating the pup, so after serious discussion with our vet put her down. No mother in early raising, no pecking order learnt.

Insect repellent works a treat.we used it as poor cocker spaniel got a real hard time. Used it with three different pups now without issue.we put it on our skin to stop mozzies. Put some on bo on ears and on neck.

if they are the only two dogs you have try to get her with other dogs especially older dogs -they will sort her out amongst themselves - the insect repellant sounds good to

Our lab has grown out of it at 12 months old or maybe one of her playmates sorted it. Was hard case seeing her walk other dogs about holding onto there collar. I like the sandfly repellant trick.

To dd to Tahrs comments, Play fight with the pup yourself so she tries to bite you, then you can be dominant with her. You are pack leader not Bo so let her know it.

Roll the pups lip over with you hand so she bits her own lip. That way they associate the biting with pain. Don't have to do it too hard, just until you get a yelp

Nice looking little pup tho

I would think Bo is not giving the pup the message as he knows you don't want him giving her the chomp. You really need to set up the pup so she starts biting Bo and you can step in and give her the "no" or "leave it" command. I would also be encouraging Bo to "get her" and while your holding her down get him to come up and sniff her and then stand over her while your holding her in the submissive position. You need to let Bo know he can discipline her then he can sort it out himself. Playfighting with her yourself can be good as well and if she starts biting you can let her know that level of play is not acceptable.

Have you thought about the fact that the pup is still young and is over stimulated and over tired. Pups are no different to toddlers, they become over tired and over stimulated easy. That’s when you get puppy biting, chewing etc
Majority of pup issues are due to this. Having free rein all day long at a really early age is not good for them. Similar things happens with kids playing with them for hours on end. They need a rest. Chuck them in a crate, kennel, whatever you’re using. Pups that learn calm from a young age are worlds ahead later on when it comes to any behavioural problems.
This isn’t a dominance thing.