Is it advisable to apply for a new passport and transfer a volunteer visa for my girlfriend in Thailand?

She need to renew her passport or get new passport first, then after that bring the paper in immigration if she will extend her visa Sir, She can use first the old passport then after that if you get already her new passport, can go to immigration CW to transfer the visa stamp into new passport, renewing passport it takes more than one month to get the new one. FYI renewing passport of Ph is before 6months expired sir.

I would say she probably has an extension, and not a visa, which can be transferred to the new passport.

Get a new passport...

When is the volunteer visa ending date ?

A person can not be in Thailand with an expired passport. She needs a new passport. I recommend getting a new passport as soon as possible

>is transferring the visa doable?

If she has a visa ("the sticker") she'll have to use both passports until she gets a new visa. If she's on an extension of stay, she can go to her immigration office and get the extension of stay transferred