SEP for a Sched C filer is deductible on their 1040. But it seems your client is an Employee, which means the retirement funds as listed on the W2 are already taken pre-tax. You don't get to deduct them, again. It isn't clear what the K-1 is from, specifically, which type of entity. And it also isn't clear if the SEP is from that W2 employment or not. All of this needs to be understood.
I recommend reading up on the retirement options. Start, for instance, with the differences in Accounts, then find out your clients' retirement plans offered by the employer and what they are participating in. There is no one Right answer to such as vague question.
Try this:
https://www.investopedia.com/ask/answers/08/roth-traditional-sep.asp
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