why second guess asking such an innocent sounding question?

...unless you have less than innocent intentions.

If she refuses it is up to her, but no harm in asking. I think you should ask. You are looking out for her at this point. I just recently did that on an extended date. At the beginning of the date the envelope was on the table. She grabbed it and put it in her bag without saying anything. We had a couple of play hours in the room, then were plannning to go out, and I asked her. "Would you like me to put your envelope in the safe while we are out?" She reached in her bag and handed it to me, again without saying anything. We had several hours of outside fun including a meal, then came back to the room for a few more hours of play time. When we were ready to say our goodbyes, I opened the safe and pulled the envelope back out.

I'd just ask her, before hand how she wanted her donation? She may want to carry her money with her, or she may want to lock it up. She's a grown woman I think she should know before hand how she wants to handle her money.

I'd definatly ask her. At the end of the day, providers make a decision to trust us and vice-versa.

I'm assuming she's a well reviewed, established provider. In that case, I'd probably ask her if she wants to set the combination.

Certain hotels offer safety deposit boxes at the Reception desk

Cheers,
K

Actually the last time I did a weekend with a gal she asked me to put the donation in the safe before I even suggested it.  I can't see why any lady would have a problem with this especially if you have seen her before.

I think I would ask if she has anything she wants to put in the safe while you are out.  Do not specify what she might want to secure.  
I've never felt comfortable leaving valueables in any hotel.

once I left money in my room and the turn down service found it and immediately locked the room. So I came back with my Bo after dinner and my key didn't work. So I had to go downstairs and ask access to my room to be told that when they find money in a room the maid service is to report it right away and lock down the room so the money doesn't disappear; just their policy. Pretty embarrassing situation.