So my motherboard is dying and I'm planning on getting the same one again. My question is will i have to install a new OS again if i replace it with the same motherboard.
I don't think you would need a new OS.
Windows is on the HDD not on the motherboard.
but he is changing a major peace of hardware so if it is windows he may need to buy a new or another copy of it to activate
Replacement for a broken board, with the exact same model should only involve possibly a call to MS to authorize reuse of that same license key with the new hardware.
MS is not that evil, and they understand parts break and need to be replaced.
Now...if you were building a whole new PC, 'just because', that's a whole different story.