I disagree. A good time to invest is after some huge bankroll dump. As there is a big chance the winner keeps playing (eventually loosing all he won).
On the other hand there is also chance he's going to keep winning and withdraw his funds ... so nothing is 100% sure. In addition some casinos like crypto-games charges divest fee for short time investors ( to avoid such speculators ).
I don't think you should look at investing in a casino with such a short-term view. Just as you said there is no guarantee that high roller will not just withdraw his winning and never come back. I would want to give an investment a minimum of several months to return anything worthwhile. Trying to time on when someone's had a winning streak isn't really anything more than taking a gamble.