It's a gift. Neither spouse *has* to provide anything at all.
I
would suggest that your partner deals solely with his child (who at 27, is
an adult) Tell child what he can afford / is willing to provide, and on
what terms (i.e. is this a straight gift, is it a loan, is it an inventment
in the property)
Don't ask the 'child' what his/her mother is
providing. If 'child' tries to raise the issue then simply suggest that
s/he discuss with his/her mother what she i able to giev, and restrict
her/his discussions with your partner to what heis able to offer.
Child's mother can do the same. If the contributions are unequal, so be
it.
If the total amount is less than child hoped for, so be it
- he or she will need to re-think their plans as to what they can afford by
way of a property.
Of cours, it would be open to your partner to
suggest a
mediation meeting to try to discuss this with
his ex, but to be honest I don't see why what your partner gives (or lends)
to his child is anything to do with his ex, or that what she gives (or
lends) is anything to do with him.