$110 is just the penalty?  Because if the balance due is under $1,000, there should not be any penalty.   It is an open secret that IRS does not assess the penalty if it is under $100, so in those cases I check the box for "Let IRS figure it."  For $110 I would probably do the same thing, because you never know when they will raise the allowance.   I don't think they charge interest and late-payment penalty on the estimated-tax penalty, but how much would it be anyway? 

Just let your client know to expect the bill a few months after filing.  It will be good therapy for paying attention to withholding requirements.