Based on the situation described in the question and options available. The change management process is very peculiar and would be pertinent ot the situation given.

Change Request rejection would be a very bold step taken by peoject manager and as per PMIism it should be the answer. Again you need evaluate and see if it is really big impact, then you can suggest customer that it should be considered as new project as this has no mention in the project charter. When you are talking about project charter, basically it is talking about the project objective & looking at the project constraints mentioned in the charter. Any in line with project objective, you may have to condier again you need to evaluate the impact is bringing on all the project constaints Cost, schedule, quality, customer sat etc. If one of the project cosntraint mentoned in the charter has been mentioned shoudln't cross this budget or schedule, then it would be easy job for PM to reject after he does his evaluation.

Before rejecting upfront yoyu may to run your evaluation with your Sponsor, so that if you are unable to convince you can escalate to Sponsor to have dialogue with customer.

This is my thought process keeping the PMIism in light, else in real world it doesn' happen like this and you still need to follow PMIism for PMP exam.

Note: Sitaution will decide the appropriate answer for your query.

Regards

CN Patil