Unemployment Assistance (as opposed to Unemployment Benefit) is means tested. So if your friend's means increase to the tune of €45, this would have a €45 impact on the amount of assistance received.
On the other hand, if your firnd is on Unemployment Benefit, they would simply lose payment in respect of the day they're working.
In either circumstance, your friend should check the impact the arrangement would have on their PRSI credits as to lose credits might impact on their capacity to make welfare claims in the future. Their local welfare office should be able to advise.