Do not give up on someone just because they are going through a hard time. That instinct to stay connected is worth following.
People in prison lose contact with friends faster than almost anything else in their lives. The stigma, the distance, the awkwardness of not knowing what to say, all of it causes people to quietly disappear from an incarcerated person's life one by one. By the time someone has been inside for a year, the circle of people still reaching out is often very small. Being one of the people who stayed matters more than you will probably ever fully know.
A letter does not need to be profound or perfectly worded. Write about ordinary things, what is happening in your life, something funny you saw, a memory you share. Normal conversation from the outside world is genuinely valuable to someone whose days have become monotonous and isolated. It reminds them that life is still out there waiting and that they have not been forgotten.
If you had a good bond before, that foundation does not disappear because circumstances changed. Friendships that survive incarceration tend to come out stronger on the other side, built on something more honest than the surface-level connections that could not hold up.
Send the letter. InmateAid makes it easy to get something to them quickly and without your home address on the envelope. Then wait and see, as you said. The simple act of reaching out may mean more to your friend right now than you can imagine.