The last thing I'd do would be to talk to his mum.
I would be pretty careful around all of this as you risk alienating your DD further.
I say that because at her age, it's easy for that resistance etc that you've been seeing in her recently to get way worse.
I'd start by looking at the basic scenario:
Your DD and this bf sleep together, are physically close, and if they haven't had full sex yet, they will do.
You won't be able to stop this and nor should you. They are 16. It's their choice and right.
This can happen with a fight with you, with ultimatums (likely to be broken), with dissent and unhappiness and your DD feeling you don't understand and somehow don't respect them and making them feel awkward and yet still closer together (against the world... aka you).
Or you can take the next step on the journey as a mother and accept it. And work with.
If he was bad, dangerous, criminal and a host of other things that might harm your DD, I'd take a different line. But he isn't. Luckily, he's lovely. So the relationship is good, she's better off with it than without it. It will be stabilising and a source of comfort and happiness. She will be much nicer with it.
In this scenario now, I would say to them in the morning:
Look, you guys, I understand. It's ok. I understand it was too hard to make you not be together in the same house. So from now on, I don't mind. When he comes over, you can stay together.
That's it. And just accept it and be grateful that she has a nice bf and is learning in a controlled and safe way how to be a young adult woman.
I'd make sure contraception was sorted.