I understand the situation, however, the panels being old will only make it even more difficult to successfully remove the graffiti.
The paint will have soaked into the panels if they have not been regularly treated with a preserver.
Also, anything you do will almost certainly change the appearance of the panels, it would be worth telling the customer that it would be better to make the change look intentional and not an attempt to blend it in which will look (in my opinion) bodged!
Sometimes the customer is not right I'm afraid and if they are not happy with the results then guess who will be to blame... you my friend.
Offer your professional opinion and if they don't take it on board then at least you gave them the right advice.
Make them aware of the possibility that it will look worse if they try to do it 'on the cheap' and if they go for that option then you are in the clear.
Just my opinions.
Good luck
Andy