OP - when you have time - & I mean enough time to process your feelings as well as the long read - you might find this thread useful -
www.mumsnet.com/talk/am_i_being_unreasonable/4460396-Youngest-Daughter-in-bits-on-phone-advice-please?postsby=Icecreamlover63
It's 18 pages, but if you click "see all" on OP's posts, you could start with just her 4 pages of updates, & see how events & revelations progressed.
Like yours, it is by the mother of a DD with a gambling addicted partner.
Like your DD, this daughter viewed her partner as essentially the same man she loved, but with an issue which was separate to him.
However, the addiction is NOT separate from him.
It is part of his identity, & (a bit like Gollum & The Ring) will slowly encroach on his personality as the addiction takes over & begins to alter his behaviour for the worse.
Right now, all anybody knows is that he's "only" pissed away his own money.
I doubt DD knows the full story yet. As PP suspect, he could have been gambling for years, & is far more entrenched than a one-off "mistake".
DD needs full disclosure - his bank statements, the savings account statements - so that she can see for herself how long the pattern has been in play.
I hope you find that distressing thread useful, & will be able to show it to DD, along with your reassurance that you understand she still loves her b/f, but you love HER, & want her to be forewarned & forearmed about how events might progress. That you will continue to love & support her, continue to welcome her b/f to your home, but that SHE needs to be fully informed about the severity of behavioural changes that often accompany addiction.