Can a Ziplock bag go in a tank?

[DreaminginBlue]

My friend has a sick betta and we are still trying to treat him. She thinks it might be bacterial, so she bought him antibiotics. He likes in her tank, as it is warm, and she doesn't want to take him out and have him in a big glass bowl just to treat him, so she wanted to put him in a ziplock bag inside the tank so his water doesn't get in with the other water? She doesn't have a quarantine tank but she wants to treat the fish.
Should she do the ziplock thing or just put him in the bowl for treatment and then put him back? I honestly don't know which option is better. Should she maybe use a floating Tupperware container?

 

[Fresh2o]

I vote for the floating Tupperware container. However, dosing the proper amount of meds in such a small container can present a problem.

 

[DreaminginBlue]

Thanks for the advice!
And yeah, it's going to be tricky...
The meds are packaged to treat five gallons, so we'd have to carefully get a small portion of the meds and dose him...
He's going to get some frozen daphnia I bought him which might help, I ran to the store today to grab some fish food and picked up some daphnia cause my friend was out of it. I've heard it's good for healing fish (or something?).
Anyways, I just hope he gets better. We thought it was swim bladder. Treated for that, nothing. Thought it was constipation, fed him a zillion peas and fasted him and... nothing. She asked LFS who said either it's a tumor or a bacterial infection. He's had it for weeks, and when I went to visit her I noticed how bad he looked, so now it's my mission to save this little guy.
Poor Bubbles, I hope he makes it.