@futurebird hard agree on the hashtags. a good intro post (pinned and tagged as "introduction") with all the tags your friend is interested in can do a lot, as well. plus, being open to following back.
your feed fills up faster than you think, so knowing that you can make Lists and put follows into different categories (news, tech, art, etc) or Mute people for varying lengths of time will make the overwhelm of a new platform easier.
that's my technical advice; this is my mental advice (feel free to ignore lol):
i don't think mastodon is a one-to-one switch from Twitter/Twitter-clone like a lot ppl think; and, they're not going to find all of the ppl they used to follow on here either. better to accept that than have false pretenses.
like, even if you find someone you know, you might not reach them because of the instance you're on blocking theirs or something. it's not one huge pot of Users split into different Areas ("the [insert trait/interest] part of Mastodon) that you can pull from. and it's not going to ever be. not designed that way.
Mastodon is a technology more than platform, so it has all the benefits and miseries of trying to join social groups and gatherings irl. all the personal politics that comes with that.
instead, your friend should consider how they interacted with their follows/mutuals. whether those are genuine connections they want to maintain or interactions they can recreate with other folx with similar thoughts and posting habits.