I have a 200 amp direct bury wire and I want to branch off it with a 100 amp direct bury wire. One electric guy said OK but I am not sure

Let me try to understand what you are saying. First you are leaving out quite a bit of information. You have a 200 Amp cable that you want to connect a 100 amp cable too?

Yes you can do that, however, the maximum amperage that could be carried by either cable is roughly 80 Amps.

Your limiting factor is the smallest cable in the circuit, in this case the 100 amp cable.

This type off connection would be governed by the "Tap" rules of the National Electrical Code. Look at 240.21....
If you really want to do it right you will need to review this or work with someone who is familiar with the rules.