This. Also what 21% said.^ This has been my experience with INFJs -- primarily with my INFJ mother but also with INFJ friends.
Two things that may or may not help:
1) From my experience, INFJs compartmentalize particular people for particular sorts of conversation. Everyone does this to an extent, but when INFJs need someone to talk to, they aren't going to do what an inferior Fi user like me is going to do, which is to almost exclusively go by vibe/gut feeling, re: how safe and trustworthy a person is. They're going to think, which friends will be able to cope with this? Who will be able to listen when I need them to, without being insensitive or trying to go into problem-solving mode? When you have an INFJ friend, they may choose you or they may not, but in the end that's their decision based on how they evaluate your strengths and weaknesses. With INFJ relatives, they may include you even if you aren't going to deal with it the way they want -- but they'll train you to react the right way, because you're family and it's your job to be there for your family. That's what happened with my mom, my dad, and me. (Both me and my dad are problem-solvers, so my mom outright told us what she wants from us in particular situations. It's mostly worked. )
2) INFJs -- and correct me if I'm wrong, since this is a recent thing I've noticed that may not be 100% accurate -- make all decisions, realizations, and internal changes on their own. There's almost never a moment when someone tells them something and it changes their mind. The way they process, is to take in all input, as well as their own internal musings, and then make their decisions/changes based on that collection of trends/data/impressions. So if you're trying to convince an INFJ to do something, or change their mind, the best you can do is throw your best argument into the well and wait for them to make their own decision. Kind of like buying a lottery ticket and then waiting to see if you won.
My point, with the above, is that there isn't much you can do besides what 21% suggested: telling them outright that you're there for them if they need you, and checking in once and a while. It's completely their decision, besides that, and there's no real shortcut.
Just being present as a friend, without smothering, & being able to not take gaps in communication personally can go a long way, ime.