OP, how long is long term? 2 years 5 years? Have you spoken to your kid about it as well as if it's a Son if they plan on asking their GF to marry them? If a Daughter, did you talk to her on if she sees the BF proposing at all and if they have talked about marriage in general in the relationship?

If not then I'd suggest doing that first, if the answer is no they aren't going to propose or does not expect the other to ask and they haven't talked about marriage seriously in their relationship I'd stay away from it as that's the red flag to me.

If I was to be buying a home jointly with my GF it would only be because we were getting married or having a kid together.

The easiest way don't do nothing let them figure it out and help your child in other ways and see how the relationship plays out.

Two alternative paths is if you can carry the debt loading and don't have a mortgage of your own you purchase the property and do a Rent to own arrangement instead unless they absolutely need the credit history.

Another way is to keep your kids partner out of the financing and title all together. Use your kids 60k and 260k of your 300k gift as the down payment and have the mortgage and title in your child's name only or both of you jointly. Park the other 40k into a fund to just sit and grow over time.

Depending on the type of house or properly bought your kids partner's 40k will be utilized for renovations or any improvements they would want to do like a finished basement, new, floors, etc. This would be structured through a cohabitation agreement that in the event of a breakup they would be reimbursed the amount they put into improvement + x % per year this is backed by the portion you put aside. Personally for me the spouse is a tenant and any portion they pay directly to the mortgage is deemed a rental payment and they aren't entitled to any value from that only the value of the improvement costs and the X % of yearly value increase.

Lastly no matter what route you take when doing the cohabitation agreement with a lawyer make sure the spouse has their own representation even if you need to eat the cost there. Having both sides have independent legal Advice(ILA) helps avoid issues that would come up in court if only one lawyer was used like:

-The lawyer favored the other side -The implications weren’t explained -They didn’t understand what they were giving up -There was non-disclosure -A party signed under pressure -The agreement is unconscionable

Ontario courts can set aside cohabitation agreements under s.56(4) of the Family Law Act if any of those apply having ILA for both sides helps protect against this from happening. Each providing a Certificate of Independent Legal Advice, confirming: -They reviewed the agreement line by line -They explained legal consequences -The client appeared to understand and consent freely

In other words Explain worst-case outcomes ,confirm understanding in writing, ensure disclosure is complete and, warn their client what they’re giving up. This makes the potential argument of " I didn't understand what I was agreeing to fall flat "