Does comparing a pointer that has been free'd invoke UB?

This seems to be a fairly common pattern e.g. in hexchat (may not compile, see also plugin docs. also note that hexchat_plugin_get_info hasn't been used in forever so I'm omitting it for simplicity):

static hexchat_plugin *ph;
static int timer_cb(void *userdata) {
    if (hexchat_set_context(ph, userdata)) { /* <-- is this line UB? */
        /* omitted */
    }
    return 0;
}
static int do_ub(char *word[], char *word_eol[], void *userdata) {
    void *context = hexchat_get_context(ph);
    hexchat_hook_timer(ph, 1000, timer_cb, context);
    hexchat_command(ph, "close"); /* free the context - in practice this would be done by another plugin or by the user, not like this, but for the purposes of this example this simulates the user closing the context. */
    return HEXCHAT_EAT_ALL;
}
int hexchat_plugin_init(hexchat_plugin *plugin_handle, char **plugin_name, char **plugin_desc, char **plugin_version, char *arg) {
    *plugin_name = "do_ub";
    *plugin_desc = "does ub when you /do_ub";
    *plugin_version = "1.0.0";
    ph = plugin_handle;
    /* etc */
    hexchat_hook_command(ph, "do_ub", 0, do_ub, "does UB", NULL);
    return 1;
}

The line in timer_cb causes hexchat to compare the (potentially free'd - definitely free'd in this example, see the comment in do_ub) pointer with another pointer, if you follow from here (plugin.c#L1089, hexchat_set_context) you'll end up in here (hexchat.c#L191, is_session). To invoke this code, run /do_ub in hexchat.

Relevant code:

int
hexchat_set_context (hexchat_plugin *ph, hexchat_context *context)
{
    if (is_session (context))
    {
        ph->context = context;
        return 1;
    }
    return 0;
}

int
is_session (session * sess)
{
    return g_slist_find (sess_list, sess) ? 1 : 0;
}

Is this sort of thing UB?

Using a value of a pointer after the object it is pointing to have reached it's lifetime end is indeterminate as stated in the C11 Standard draft 6.2.4p2 (Storage durations of objects) (the emphasis is mine):

The lifetime of an object is the portion of program execution during which storage is guaranteed to be reserved for it. An object exists, has a constant address, and retains its last-stored value throughout its lifetime. If an object is referred to outside of its lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when the object it points to (or just past) reaches the end of its lifetime.

And using it's value (just for anything) is an explicit undefined behavior as stated in Annex J.2(Undefined behavior):

The behavior is undefined in the following circumstances: [...] The value of a pointer to an object whose lifetime has ended is used (6.2.4).