Both matter, but if you are asking which one to prioritize when resources are limited, the answer is financial support, and here is the reasoning behind that.
Money on the books is tangible. It buys food from commissary that makes the daily reality of being locked up more bearable. It covers phone time so the connection between you stays alive. It pays for hygiene items that the facility provides at the bare minimum. Every dollar on that account is a concrete expression of care that your person can see and use every single day. In that sense, financial support is also moral support. Showing up with money says I am here, I have not abandoned you, and your comfort matters to me, without a single word being spoken.
A moral support speech, however well intentioned, lands differently in a cell than it does in a living room. Someone in the early stretch of incarceration dealing with withdrawal, adjustment, and the psychological weight of being locked up for the first time is not always in a place to receive and hold onto words the way you hope they will.
That said, do not abandon the moral support entirely. Letters matter. Visits matter. Knowing someone believes in your ability to come out the other side of this is something that accumulates over time even when it does not feel like it is landing in the moment.
The other thing worth keeping in mind is that this incarceration, painful as it is, may be the intervention that finally creates enough disruption to break the addiction cycle. A heroin addict who is forced into sobriety by circumstance has a window that does not always stay open on the outside. Encourage every treatment program and counseling resource available to him inside. The financial support keeps him comfortable. The programming is what gives him a real shot at coming home different.