I have SBC DSL, and I have DSL upstairs for my computers plus I have a laptop downstairs that can use the dial-up connection. So if a relative comes over and needs internet access, we both have access simultaneously.
As for your question, I personally don't think they will care unless your friend abuses it. Plus, according to SBC, you can only have one DSL connection and one dial-up connection active at any one time. So if, for example, you're traveling and need internet access, you can't use a SBC dial-up connection if your friend is using the dial-up connection and your DSL is on at the same time.