I am currently working as a junior C# developer in the Netherlands. Last Wednesday, a senior developer, who was working for a client I never met, went on a summer trip for 4 weeks. Before he left, he did some changes to a project for that client and made it ready for deployment on production. We work with a schedule and always deploy our project to production on Thursday. Because he wasn't working that day, he asked me to only finish the deployment and mail the customer if the deployment succeeded.
So, I deployed it, fixed some issues that popped-up and informed the client that it was done. The next day (Friday afternoon) I received a mail with a long list the client want to see changed or added before he leaves on his holiday next week Friday. In his mail (which wasn't written in a very friendly way), he wrote about how he "asked a dozen times to place X on the left","made clear he wanted to see Y replaced with Z", "How hard it could be to remove A,B and C" and referred to different mails he had sent to the senior in the past, but never to me. (Some of the mails he referred to were even sent before I started working there).
At first I got the feeling he confused me for the senior, but the client did add him in CC himself so he might have done that on purpose too.
Either way, I doesn't seem possible for me to finish all what he asked in such a short time. I have 2 days off next week and still need to finish a lot of work/documentation/bug fixing etc on my own projects before I leave for my holiday too. Besides of that, I don't know how the project of the senior was built and he doesn't seem to have any documentation about it. He was also quite confident that everything would work perfect, so he never really told me which actions to take when I asked him what I should do when the project failed.
My question here would be: How to handle a situation where the client is asking you to change a big part of a colleague his work, without the colleague knowing about this?
I thought about just mailing the client back and saying I would inform the senior about this since it is his project and knows how it is build, what was agreed on etc, but on the other hand, I don't want to say "no" to a client without talking about this with my manager first. Then again, my manager is a very ambitious person and would propably just tell me to do it either way, which wouldn't be very liked by the senior who doesn't want people to touch his work at all...
I hope that's enough information to get a clear view of the situation. :)
Edit, Extra information in case someone asks: I received his mail just this Friday afternoon when I came home from work.