Don't you think that's a bit harsh? OP wrote a single paragraph, that's not enough for us to know how they interact on a daily basis. Creating the dependency doesn't seem to have been on purpose. It happened, it created problems, probably for both of them and OP wants to find a solution that hurts their friend as little as possible. I find that highly commendable. Such situations happen, you only notice them when it's too late and usually there is no good solution. You can't just stop supporting them because that would cause serious problems for them but you can't keep silent about your own needs either unless you want things to escalate somewhere down the road.

Now, the polyamory out of desperation thing is a real problem and I know many poly people (including myself) who have at some point suspected that their "original" partner has only accepted this lifestyle to avoid losing them. And let me tell you, finding that answer is hard. If you don't ask, you might never know. If you ask once, you won't be sure if they tell the truth or just want to protect your relationship. If you ask too often and they actually are okay with being poly, you may annoy them. The only way to resolve that is to make sure you can openly communicate about anything and everything. All involved parties must be comfortable telling each other about their pain points and be sure that a disagreement will only strengthen instead of weaken the relationship because everyone will try to find a good solution.