I wish i could say she was right but I feel it would be wrong for me to say so.
Brace up IceCream - I'm not your DD's mum, I'm a random stranger, & it's blindingly bloody obvious that DD is right, & that you are not wrong to say so.
DD's own MiL knows her son is wrong - she's furious about his gambling, remember?
Your son in law isn't just gambling - he's actively stealing from, gaslighting & abusing your girl. Pull your big mum pants up & get usefully angry. DD is going through an extreme situation & you are ALLOWED to be 100% partisan. OK?
btw, your son in law's commanding officer would not be impressed to learn that one of his soldiers is stealing from his own wife FFS. Keep that up your sleeve for now, but it's worth bearing in mind if son in law decides to kick off with more gaslighting & power plays.
Just be a warm & comforting presence for DD today, & let her talk herself out. She's got a lot of bullshit to sort through, & will be feeling confused & undermined. Stay tough, stay calm ... but let your inner tiger roar. That man IS in the wrong & you ARE permitted to state that fact.