SEP IRA Union Exclusion

My client owns a small electrical control company (S-Corp). He wants to start a SEP IRA to make contributions for 2018.

There are three parties involved:
1. The owner of the S-Corp, currently a union member
2. The owners wife, works very part time as an office manager
3. One employee, covered by the union retirement benefits

Question: Is the owner able to exclude the union-covered employee and still make contributions for himself and his wife up to the applicable limits?

Even though he is also a member of the union?