The past tense of let someone stew in their own juice is also let someone stew in their own juice.

The third-person singular simple present indicative form of let someone stew in their own juice is lets someone stew in their own juice.

The present participle of let someone stew in their own juice is letting someone stew in their own juice.

The past participle of let someone stew in their own juice is let someone stew in their own juice.