A lot of assumptions are being thrown about on here. OP, does not explain the circumstances of her move to be nearer the 60-something friend. Was she explicit - eg,"I am moving to be near to you?" Did the friend have any choice or part in the matter? Or were the reasons for the move kind of 'fudged' as needing to be nearer to shops or other conveniences, or a place with no stairs etc etc? We don't know. Perhaps the friend was freaked out by the OP moving to become more dependent on her - and perhaps this is the very reason that they plan to move away and are being vague about where.
OP states she doesn't care much for her friend's DH. Could it be he is annoyed his wife is being sucked into a role as a carer and he wants her to maintain her boundaries around this? Is this why OP is less keen on him?
Whatever the case may be, as if anyone would reasonably expect a man they don't even particularly like, to stay put on the basis you may need his wife around!
These people may well have their own elderly parents to worry about. Even if not, they will have had years of the 'sandwich' phase when you're perpetually caught between the demands of ageing parents and growing kids - not to mention work demands. They possibly envisage a bit of freedom in their 60s - who can blame anyone for this?
A friend of mine had a similar situation where an elderly lady in the flat below her became very dependent on her - to the point she made it very difficult for my friend to move to a larger house after she had her twins. This kind of situation is very difficult indeed, for all concerned.
OP, I think all you can do it let this play out. Don't try to manipulate the situation. They may take ages to move. Regardless, don't use your will as a bargaining chip. If you maintain good and fair relations, I'm sure your friend will see to it that you're ok - eg she'll have you to stay frequently and / or help you into a supported living situation perhaps where you will be surrounded by a community.
Rven though I have kids, when I'm your age, who knows where they will be? They might be overseas or anywhere. I can't assume they will always be local because people need to live their lives,