Mehndi and validity of wudhu
In the name of Allah, the most Beneficent, the most Merciful.
Mehdi when applied on the hands or hair only gives it a colour and does not put a thick layer over them; therefore it does not prevent water getting to the hair or the skin.
With regards to your question, the wudhu will be valid and you will not need to repeat the past weeks salah. (Masaail Wadhi – Mufti Rafat Qasimi p.91)
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham