Well, I have listened at length and talked at length about this sad situation. I think personally I would tell her that you love her and miss her, but can't live waiting to hang out with her again and are moving on until she reaches out to you. Then I think I would tell her that you have a responsibility to your son and to her son to help nurture their friendship until such time as they decide they aren't friends anymore... therefore would it be okay if you had her son over every once and awhile for a sleep over (of whatever). That way you can still have a relationship with him and so can your boy.
My concern is the lack of attention and the abandonment issues she is creating in his life when she repeats this cycle of pulling people close to her and then ditching them. You don't need to perpetuate that for him. You can still be there for him and be a constant. You don't have to abandon him...
Next time you have him over to your house maybe you could talk to both boys and tell them that you are doing your best to make sure that they get to hang out but that adult relationships are complicated and sometimes that is hard to do. Her son might think it's him that is to blame for not being able to see his friend and that is not the case. Kids are smart, but naive. They need as much information as possible to feel like their world is safe. Sure things sometimes can't be changed, but if we talk to them about stuff then at least they know it has nothing to do with them and can rest easy and adjust.
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