Different tenses: Over the years, he has volunteered... He helped... He thinks

(ex) Over the years, he has volunteered in a lot of different schools. He mainly helped students who struggled with English and Math. He thinks his volunteer experience is very valuable.

My non-native English speaking friends think I should use the present perfect "has helped" instead of the simple past because it is related to the volunteer work he has done over the years. But, I think I should use the simple past "helped" because the same verb tense is used for "struggled" in that sentence.

Which tense is correct for "help": the simple past or the present perfect? I really appreciate your help.