If a President groped someone, and she fought back (e.g. by kicking him hard), how would the Secret Service react?

SS, both agents and uniformed, protect the president, no matter what. Other agencies would take action.

If there was a kick the president would be stopped because he would b e swarmed by agents for protection. The kicker would also be stopped and detained. Like real police the SS does not charge, that would be up to the US Attorney's Office when it came to the person kicking.

As to the president being charged that is up to congress with the investigation into the president's actions by whoever teh US Attorney's Office and the Congress assigns to the investigation.

The president can also be sued for non presidential actions

Ask Paula Jones,Gennifer Flowers, Kathleen Willey, Juanita Broaddrick and of course Monica Lewinsky.

Presidents cannot be arrested, or sued while in office.

The person would either have to get Congress to Impeach or wait until the person left office.

The secret service protects the life of the president. They don't generally protect him from bruises.

It won't happen so don't worry about it.