Can thief unlock stolen phone that was locked by Android Device Manager?
The answer is almost certainly yes. 

Someone could probably get past it in one way or another. (They could probably just use odin on a samsung device for example. Though I have never done something like that myself.) If you have sensitive data on there then a remote wipe might be the best thing.

Did you report it stolen with your carrier? 
They could report the ESN / IMEI  as stolen so at the very least the thief probably wouldn't be able to sell it.

Of course I do not know your exact situation. If you know for sure it was stolen and not lost could the police recover it from the thief?

Either way best of luck to you.