Resignation Letter as evidence

This inquiry is on behalf of my friend.

My friend has been working for their company for almost a year. Recently their company hired a new Director. The new boss was very harsh them. Minumura, laging nakasigaw, namamahiya. Apat sila nagresign dahil sa sobrang hirap pakisamahan ng bagong boss. My friend's resignation letter was different among his other colleagues. Nilagay nya sa resignation letter na hindi na nya kaya 'yun pagtrato sa kanila. Na nahihirapan sya kasi lagi nakamura at nakasigaw un bagong boss. Nadescribe nya un work situation nila sa letter. Immediate resignation un request nya at pinirmahan naman nung boss.

Lahat ng nagresign balak kasuhan sa DOLE 'yung boss nila. Pwede po bang gamiting isa sa evidence 'yung resignation letter signed  by their boss and noted by the HR?

thanks!

yes, it can be used as evidence.

thanks attyLL.. a follow up question..

right now all those who have resigned are waiting for their back pay. They're not yet filing their cases in DOLE because they're scared that the company will hold their back pay.

*if they submit their cases to the DOLE can the company hold/freeze their backpay?

*From the time na ginawa 'yung offense to the time na irereport sa DOLE, may time limit ba na kung hanggang kelan valid un report? (ex. the offense should be reported within 30 days.. hope my question makes sense)

again, thank you very much.