If you haven't already, remove it from the frame as soon as possible. It can get progressively
worse.... and hard creases can form....
Art on thin Japanese papers do not react well to Acrylic directly on the face,.... I learned this a long
time ago the hard way, as you are learning now.
At 15 x 57, I would reframe with glass and spacers. You cannot use regular acrylic with spacers
because of the inherent static charge , your only option in acrylic would be Optium, which is $$$$
Removing ripples- you may get lucky and they reverse by getting it out of the frame, if not flattening
can be a hands on invasive procedure best left to conservators, too many variables here to give advice
except to say don't experiment on the customers art.
Also ...in general ....some rippling/cockling etc. is normal and desirable for this type of art and is an inherent result
of the painting process.