Their ranks mismatch, so he can't provide the quality guiding needed to bring him back from near overload
Despite mutual feelings as a B rank guide he cannot meet the ML's guiding needs because he is an S Class (imprint causes the Esper to receive guiding only from the Guide they imprinted on) so this is dangerous for mostly the Esper with the Guide unable to provide full guiding without passing out or being hospitalized and the Esper if not able to get appropriate guiding he will get corrupted and possibly die so undoing the imprint is what was recommended to them. With them undoing the imprint the ML then can get guiding from higher classes but undoing the imprint has consequences like death so they don't want to undo it until now.
Huh? Im confused as fuck why do they have to undo the imprint again? Aren’t they mutuals in feelings alrdy?