How to respond when she says she has a boyfriend

She didn't have to mention it. The fact that she did means she's either shutting down romantic interest before it builds, or she's drawn to you enough that she wants to be clear about her limits. Both of those are useful information. Some women mention a boyfriend early to filter. Others mention it later because the interaction got flirtier than they expected. Either way, she's not angry. She's managing the situation.

Read the context. If you've been light and funny, she's probably just setting a boundary while staying friendly. If you've been heavy or explicitly flirting, she might be softening a rejection. Your reply needs to work for both, so keep it neutral and unbothered.

First, acknowledge without making it a big deal. Don't ignore what she said and don't act like she crushed your dreams. A one or two sentence response that shows you heard her and think it's fine.

Second, give her something else to do. Don't end the conversation on her boyfriend statement. Say something that invites her to keep talking about literally anything else. A question works best. This proves you're not wounded and keeps you in her world as someone interesting to talk to, not someone who needed managing.

Reply within an hour or two. Waiting days makes it look like you're overthinking it. Fast reply plus a light tone equals you handling it well.

After you send, keep talking to her the same way you were before, or slightly lighter. Don't avoid her. Don't pretend she doesn't exist. Don't bring up the boyfriend again. If she stays engaged in the conversation, you're fine. If she stops replying, you have your answer. Either way, one reply settles it. Don't send a follow-up explaining or clarifying.