In real life there's very little difference. The first sentence is absolute. It's a guarantee. You are here to open a bank account today. You expect to get it done right now. I know that Mr. Xyz does that. I see he's in his office and he's available. "Go through the door to your left. Mr. Xyz will assist you in this issue." The second expresses some degree of doubt, or some degree of possibility that things could go in more than one way. You are curious about Roth IRAs. You have questions. I don't know the answers. Who can assist you? Let me think. Yes, Mr. Xyz does all the investment things, and I think he does Roth IRAs. And I think he's in today. He would be the right person to go to. "Mr. Xyz would assist you in this issue."

"will" implies knowledge that Mrs X can and will help. "would" implies an assumption. She might not know how to assist, or might not be able to,or might not even want to.