alinrj you could communicate between the 2 apps using the REST API https://docs.flarum.org/rest-api
The lastSeenAt value will be visible on the GET /api/users/<id> endpoint. If you access the API using an API key / admin access token you should even be able to see the value for users who chose to not publicly disclose it.
If the 2 apps are on the same server and you are only reading the value, you could probably use direct database access as well. It's not recommended to use direct database access to write to Flarum database as it would bypass middlewares and events, but for reading it's fine.