Unusual situation...

So far my adoption story has turned out really well. I found my almost 24 year old son last year through facebook. I contacted him and for the first two weeks it was an angry kid asking angry questions like why now and such. Our first meeting seemed to go okay and then he dissappeared after we had driven two states to see him. But things turned around and after 7 months of just expressing concern and love for him, he moved in with his father and I. He has been with us almost 6 months now and it is more than I could ever ask for. His amom and him never had a really good relationship. His aparents divorced when he was very young and they have both remarried twice. He moved out of her house at 18 years old. Anyway, she has not been happy about him coming to live with us. I encourage him to keep in contact with her because after all this is the woman that raised him. Anyway....the really strange part, my son is falling for my best friend. She is 12 years older than him, in the middle of a divorce with two kids(her daughter is his sisters best friend). She has been open and honest with me that they started texting and she really doesn't know how she feels. At first I thought it was a sexual thing for my son but now he is saying he wants a relationship with her. I am not mad at anyone about it because I know we can't control how we feel about someone. I am just scared of losing one or both of them. Anyone else heard of something like this happening? Both have said they will not even think about this if it would jeopardize our relationships. Need some advice...