Be careful of who you talk to.
Coming out can be an incredibly difficult and scary process, and trans people need to be allowed to be in control of who knows what about their gender.
If someone has trusted you enough come out as trans to you, you need to be sensitive regarding who else you talk to about their identity. As a rule, you should always ask them before mentioning to anyone else that they are trans. It's also a good idea to ask whether they go by different pronouns publicly and privately, so as not to risk outing them by using the correct set of pronouns for their gender when talking about them with people who aren't aware that they are trans.
Never assume that other people in a trans person's life will necessarily know about their identity. Trans people come out at different times to different people - in some settings, groups, or relationships, they may never come out at all. Reasons for this often involve their levels of comfort and safety. It should be entirely up to trans people who is told what about their gender, and outing someone can be anything from uncomfortable to directly dangerous.
Content Warning: this page includes mentions of coming out, outing without consent.