I don't think you should get involved at all, it's their relationship.
She did ask to meet for a drink which he did, given the lack of contact on both sides, he might not have thought that she'd be interested in going for a meal with his second wife and children. Contact works both ways and if neither of them are doing particularly well at this then assumptions can be made on both sides due to misinterpretation, she in that she feels something is 'missing' and he in that she mignt not be interested in spending more time with him.
Communication on both sides is the key, and as you say your daughter is very independent and capable then it is for her to resolve or decide that it's not worth the emotional effort.