My answer would depend on for how long they will be "taxing you around."
If for several days or even if just one day, make sure to pay for the friend's meals while you all are out and around, as well as petrol...then also insist on sending the friend off with a full tank.
If they say, no, no, no, then ship something really nice (reflective of the time and expense the friend went to) once you get home, along with a sincere note of thanks after the fact..
Ya' gotta ask yourself what would show sincere appreciation if you were the one taxing........you would likely turn down on-the-spot offers of meals and petrol...but you would smile if a nice package and sincere note of thanks arrived later.
If you offer to pay for all meals while out and about, and if he/she turns you down or agrees to split only, then INSIST on having them choose a really nice restaurant for the "last night special meal" that will be "on you."
Not knowing the friend, nor the dynamics of said friendship (old college buddy, business associate, etc.) , nor if the taxing is one day or 20 days, it's hard for any of us to really advise.
If you think they might really "need" the cash, you can always hand them a sealed envelope with a card with a hand-written note, and just say: "Here is a heart-felt note of thanks I wrote out, but please read it later, as I get all embarrassed because I am not the best writer...so please read it later..............." at which time they will find the kind words and your kind gift of cash (again, if you think that is what they really need."
If the "need" is not really a serious need, then treat them to a really nice dinner somewhere they might not treat themselves to......to "create memories.'