The expression "going mental" is an informal colloquialism used to describe someone who has lost control of their emotions or behavior. However, sometimes using informal language is not appropriate or professional. In this case, there are a variety of synonyms that convey the same idea but are more formal. Some possible options include "losing one's composure," "having a breakdown," "falling apart," "going crazy," "losing control," and "having an emotional outburst." By using these synonyms, you can express the same idea in a more measured and professional way. It's always important to choose language that is appropriate for the situation and audience.