My husband replaced our roof with 30 yr GAF architectural shingles. He used Dewitt's Neo Seal Neoprene Roof Cement under some shingles (the ones in the valley, on the ridge cap and under some that were bubbling up). After my husband used it, he noticed the next day that there was a shingle that was soft, mushy and losing its grit. I read the instructions on the cement he used and realized that this should not be used under shingles! Can someone please advise us on what to do (if anything) about this situation? Thanks in advance!

Neoprene is incompatible with asphalt shingles.
You can use small amounts on the granulated surface with no problems, the granules protect the asphalt.

If you use too much neoprene on the under side of the shingle it will do what you are describing.
The only good way to fix this is to remove and replace the affected shingles, which probably means re-doing the valleys.

You may be able to just leave it alone and the neoprene will solidify, the neoprene is much stronger than the asphalt the shingle is made out of.
It is hard to say without seeing it first hand.

It is the solvents in the neoprene that are attacking the asphalt.
Once the solvents flash off the deterioration should stop.

Try lifting the cut side of the valley a little bit, this should pull the neoprene apart and allow the solvents to flash off a bit faster.
The sooner you do this the better, if it feels like it is firmly stuck stop pulling at it, you want to be careful not to crack the shingles at the nails.
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Thank you so much for your response. It really helped. My husband is on the roof right now replacing the affected shingles.

There must be something else in that type of neoprene that would eat the shingles. I use neoprene all the time and have never had this happen. Black Jack neoprene flashing cement is used on composition all the time. I assume this Dewitts must have a different catalyst.