This is “MY” take. When you go about doing a “chessed” it is not a matter of your convenience it is a matter of doing the chessed. If you choose to combine it with something else that is your business but if you say you will be available to do a chessed at a certain time then that is when you are on the clock for that chessed. If you are at a wedding and offer to do a chessed on the way home from that wedding, then it is your responsibility to leave that wedding on time no matter what. The other girls are either in it with you or not. Your obligation is first to the job you agreed to do and the girls at the wedding should be made aware of that. If she didn’t understand the obligation or responsibility she took on when agreeing to pick up this person from the hospital then she shouldn’t have volunteered and someone else would have. That is the point. Her “Job” was to leave the wedding on time and make herself available to the person in need on time, be courteous to that person and be understanding to them. If she couldn’t do it, or didn’t understand how its done, then she should have left it up to someone who could.
Anyone who volunteers to do a “chessed” should understand what they are signing up for. They are NOT doing a favor for someone else who should be grateful to them. They are signing up to do a mitzvah and they should be grateful for the opportunity to do it understanding that if the shoe was on the other foot, Hashem will send them their own shelichim to help them.