Answer: The Shulchan Aruch (Yoreh De’ah Siman 391 Se’if 3) writes regarding an ovel attending a chupah that some permit this, and some forbid this, and [according to the stringent opinion] the ovel should stand outside in order to hear the brochos [recited under the chupah].
The Remo (ibid) writes that the ovel cannot attend the chupah if there are musical instruments playing, but if the chupah takes place inside a Shul and there is no simcha during the chupah [this presumably refers to musical instruments or dancing] then the ovel is allowed to attend – according to one opinion after shivah, and according to another opinion after shloshim.
The Remo concludes by quoting another minhag that the ovel does not attend a chupah for the entire year [following the demise of a parent], instead the ovel stands outside Shul in order to hear the brochos [recited under the chupah].