Discarded New York City gum is amongst the toughest in the nation. A real bitch when it's old and rock hard. Do not underestimate it!!
A lot of gum can be a real nightmare, especially when you have not seen much of it. I can only imagine a theater full of it.
My first thought is, are you charging separately for it, or are you including it in the price of the cleaning??
If you have not given a price yet, you may want to rethink your pricing and offer them 2 options. It may not be worth it to you or them to attempt to remove it all.
Yea, many solvents work at softening it up, but it still involves being on your knees the whole time, working one piece at a time. A real wrist killer and very frustrating. Very tedious work when you have lots of it and don't have the high heat of a truck mount to melt it away. I have been considering a hand steamer like Danny Boy recommended, hell, maybe even one of those steam bullet-shark thingys. Anything to soften it up as heat is your friend, and it will help the solvent penetrate it better.
If you do take it on, make sure to bring a gum lacky so you can focus on the cleaning.