Player split from the party, showed up late, then got upset for not being involved - advice?

When I wrote this my English wasn't that good and it was too long so made it shorter while trying to keep main points. Not good at writing that well

I ran a session focused on RP while the party traveled by airship to a new continent. One player had told me they’d be missing a few sessions due to work but might drop in late once or twice, they gave me a timeline as well. I said thanks for the heads-up and planned accordingly.

Before those sessions, they decided to have their character split from the party and travel separately. I told them it wasn’t necessary they could just “fade out” and reappear later but they insisted it made more sense for their character. I reminded them that if they weren’t with the group, I couldn’t promise them screen time.

On the session day, they showed up two hours late with no warning and didn’t say anything in chat or voice. I called on them three times to try and give them a moment anyway on different points during the session, but got no response. Eventually, I stopped trying and kept the focus on the group who was actually present and engaged.

At the end of the session, which everyone seemed to enjoy, they messaged me upset and angry saying they felt excluded, ignored during a shared dream sequence, and hurt that I hadn’t carved out something just for them. They accused me of not balancing spotlight fairly and said that seeing everyone else have fun made them feel isolated.

I explained that I’d tried to include them, but they hadn’t responded. They kept pushing, saying even 10 minutes would’ve made a difference and that my handling of it just made them feel worse along with stating that they cant stay focus when they rent included and kept saying i didnt care. Honestly, the message felt emotionally manipulative, like I was being guilt-tripped for not bending the session around someone who barely showed up.

I haven’t replied since. I’m not sure how to move forward. I tried to be fair and communicative, but now I feel stuck between keeping the game fun for the group and managing someone who isn’t meeting me halfway, honestly I'm disappointed, this is the first time they have actually done this anger and sadness to me but looking at it, its not entirely my fault here

Any advice on how to handle this kind of situation?