I have a friend who wants to use DL miles to fly SFO-VVO, possibly with a stop in LHR.

Any tips or advice both on how to do the routing, how to search, etc?

In the past to go SFO-VVO he has sometimes had to buy two separate tickets, I think one SFO-ICN and the other ICN-VVO.

I told him that the stop in LHR is probably going to be a no-go, that he's just not going to be able to do the routing that way.

In case it matters, he is probably looking for Y. Also, I think a significant number of his miles will come from American Express Membership Rewards, so routings on other Amex transfer partners wouldn't be a bad thing.