Does 听说你去丽江玩儿了 imply they are still in 丽江?
A:听说你去丽江玩儿了？那儿怎么样？
B:那里特别美，空气也很新鲜。

From the context, we can deduce that B is currently not in 丽江 because B replies 那里特别美. If B is still there, he/she would say 这里特别美.
However, 听说你去丽江玩儿了 could be interpreted either way you suggested. The sentence only tells you have gone to/arrived 丽江, but it doesn't tell you are currently out or not. Additional information must be required to determine whether you are still in 丽江 or not.
It's clear, in your context, B is already out of the town.
PS. It's also possible that B is still on the train to 丽江, when A is talking to B on the phone saying 听说你去丽江玩儿了. But this is not the case in OP's context.