Partner has been making 401k contributions as nondeductible in error

Just found out that my partner has been making his 401k contributions as non deductible since he started this job 3 years ago. He was supposed to elect them as roth and some how selected the option below that. Most of the answers I have found about this was if it was an employer or plan administrator error. Is there anything we can do to fix this or mitigate the damage?