Weezir: I will leave for someone more experienced in Canon law the question of whether or not you can be an official witness.
The problems I see are of a different sort, and they are the sort that causes untold headaches, heartaches, and often, another divorce, and those are the problems of what is euphamistically called a “blended” family.
It was not at all clear who had custody in each instance; although if the law of averages applies, she has custody and he doesn’t. There is, however, a very consistent pattern among children, and that is, that they seek, want, dream about, and desire the reuniting of their own family. In other words, his daughter is not too young to want her daddy back home (assuming mother has custody), and her daughter is highly likely to want her father back home too.
The grief that can come of this - not only to the chidren - can be enormous, especially when one or more of the children start to “act out”. It can also occur when the two of them have another child; now we have two young ladies with a half-sibling and the dynamics that entails.
And this doesn’t even get into the issues of discipline - “You aren’t my [dad, mother] and you can’t tell me what to do!”
Add to that, the potential of one, if not both, not having really worked out why the previous marriage(s) didn’t work out, and what part they had in the failure, and you have an instant recipe for disaster. Just add “luv”.
They need some extensive counseling; for themselves, for them as a couple, and for them as a blended family, and they need it now, not after they have jumped whole-heartedly into this Pandora’s box they have opened. Believe me, it is way more serious than they have any clue.
It is not for no reason I categorize people like this as “fools and simpletons” (and I mean no disrespect). They become what I call “clients”, i.e. they end up either at the counselor’s office in disarray, or at the attorneys’ offices.
You may be told that “they have that all worked out”, or “it’s not a problem”, or “my little Mandy just loves {whoever}”. What they are really saying is that the laws of human nature have been suspended just for them and that things are “different” for them. In that case, send them to me; I have a bridge I need to sell…
They are not only an accident looking for a place to happen, but a series of accidents bound to happen over a period of time. Like Tom Shane says, when he advertises his diamonds, “I guarantee it”!
They need professionl help. The best thing you can do is get them to it.