When you go to choose one there are a lot of important things to consider. Your friend will probably be happy with anything you choose but it's always nice to find one that feels like it fits their personality and needs.
One of the first things to think about to narrow down the options is how many people do you think will ride on this, and who are they? There are towables made just for children, or ones meant for everyone. There are ones meant for a solo rider, and ones meant for up to a dozen. Narrowing down the number of riders will narrow down your selection a lot and make it easier for you to see what's out there.
The next thing to consider is how much you're looking to spend. Tubes come in a very large price range so you should be able to find one that meets your needs, but keep in mind that the more riders you're wanting the tube to hold the higher the price range tends to go.
Does your friend have a favorite color, or do they like to be a little different and stand out? You can find classic, circular, towables in every color combination you can think of so if your friend loves a particular color this should be easy for you. If they like something that really stands out there are options for that, too--for instance, there are giant hot dog or whale shaped tubes for you to choose from.
If they already have a boat they'll probably have life vests already so you won't need to be too concerned with that, but a good tow rope or harness for the new tube is a good idea. You might be able to find an inflatable that already comes with one, but if not this is something else to look around for before checking out.
It can be stressful picking out a present for someone but really it's hard to go wrong here. No matter what you choose these are always fun to use and the recipients are always excited when they get a towable tube as a gift to get out on the water and use it.