This is a tricky question, because it’s unclear to me whether this is something your friend does out of a want to do so, or it’s simply easier for them to pay, given their means, than to split the bill and feel badly about what they ordered.
Having that one friend who always insists on covering the tab is nice in theory, but can be very hard to deal with in practice. While the friend may legitimately enjoy treating you, eventually resentment can build, both on the part of the friend and you. Dealing with money and friends can often be a delicate dance, and no one wants to be in a situation where they feel the other is not appreciative, or taking advantage.
If your friend picked the place, did the majority of the ordering, including selecting the wine or cocktails for the table, and they then insist on paying, not only do I think covering the tab is fine, but it’s expected. When a person invites others to dine out, especially if it’s a fancy place, and then they order for everyone, the impression is that they plan on treating. The alternative would be extremely rude. Sometimes people just want companions to come along while they check out the new hot place, so they don’t mind covering it, especially when they know they can afford the meal and others can’t. If you have a friend like this, make sure not only to thank them, but to also invite them to a meal you can afford to cover every so often, and pick up the bill.
On the other hand, if you and your group have picked a restaurant together, and all ordered collaboratively or separately, having that one friend who always insists on paying can get to be a little much. It can start to feel as if the friend is either paying to hang out with you, or assumes you can’t afford the meal, which is demeaning. In this instance, it’s OK to insist that you want to pay, too. Tell the friend that while you appreciate them always offering to take care of the bill, you’d like to contribute. If after this, they still insist on covering the bill, thank them, and try your best to pick up a drink at the bar or the next time you’re out. Then, when you’re out together, tell them that while you appreciate them covering the tab every once in a while, you don’t want this friendship to be one where they are always paying. They should understand, and resentment on both sides will be avoided.
