I think this is one of those “it depends” situations. It really depends on what they mean by “no” in relation to believing everything the Church teaches.
There is no way for an RCIA program to fully explain everything in a short period of time (typical RCIA is about 9-12 months unless they do it right and then catechumens might be as much as 3 years). In this time period, they will be exposed to many teachings, some will be at a high level that really deserve much deeper discussion and explanation. There will be many things to absorb.
So, if by “no” they mean they embrace the Church as having the fullness of truth and authority, but don’t fully understand everything yet, aren’t sure why the Church teaches what She teaches on various topics, maybe even that they “disagree” in the sense that they don’t fully accept it because they don’t understand it-- this is normal. If they fully intend to obey the Church in all her teachings as they continue to study, learn, and embrace the faith more fully, then I don’t see anything wrong with being their sponsor.
If they mean “no” as in “I don’t believe it and never will, and I fully intend to break XYZ law of the Church” or “no, I’m just doing this to please X, but I don’t really believe it” or “I can be a Catholic and still do X” then, I would not sponsor them. In fact, I would go further and tell the RCIA director and pastor that they are not ready to be admitted to the Church. That’s the role of a sponsor.