Luke would have offered to help -- with both 9E or Ari, seeing as he also had R2 and Threepio here and both could easily keep those other droids company. And Ari and him got on very well. But it felt like it was in poor taste to try and take over something that Rey was clearly looking at like an important task.
One of which she possibly needed, considering her other concerns.
"It's always easier to find and indulge in the things that are bad for you," Luke said. The dark side was no different, really; it appealed to people's baser nature. "Is there something within the dark side that appeals to you? Sometimes, it is tempting -- my own father was unable to let go of those he was attached to, and that was his greatest temptation, the reason he let himself be seduced. But it seems to me that if you took a darker path, you'd be choosing to let go of your attachments. Even Ben has found a line he doesn't want to cross again."