chocolate two of my friends take coke. One is a genuine recreational user - he can go for months without using, it's always taken in a social setting. The other is, I think, using on a much more regular basis to get over hangovers and generally function. The difference between them is that Friend A is pretty sorted and happy; Friend B is using it as a crutch to deal with stress at work, stress at home, and a few issues of his own that he's never got to the bottom of.
I don't think Friend B will ever kick it, despite many warnings, interventions, pleas and ultimatums from family and friends, because he doesn't think it's a problem. It isn't in itself, it's just all his other problems that he's not solving, but which the cocaine is masking. The trouble is, now he's got all the other problems plus a growing coke addiction. It creeps up very quickly.
The worst thing is, Friend A is single and childless; Friend B is married with three small kids. We, his mates, are beside ourselves with concern, but his wife, like you, has so much at stake that she's constantly minimising it. Meanwhile, we're resigned to the phone call, from one of them.
Please talk to your dad. Your DH needs to deal with the reasons he needs to take cocaine as well as the cocaine use itself. Coke makes you feel invincible, but when you're a parent to small children, you're absolutely not.