Should I DM this girl?

So my boyfriend works with this girl who likes him a lot and he said one night she spent the night at his house because she was locked out of her dads house and that she tried to sleep in the bed with him. He claims he said no and that he told her they couldn’t do this and all that bullshit but today I see that he liked and commented on her picture being flirty? Should I dm her and ask her if they are together because I don’t believe him anymore and I just want the truth. Or does this sound crazy? (I don’t know her at all but I know about her)