[addons_zz]

Only if he is a quantum player from the quantum computer games, where the quantum bits can be 0, 1 or both at the same time.
But speaking about SrcGold games, the player cannot be alive if it is not connect unless some terrible bug on the AMXX.

But I just in case, I checked the code:

static cell AMX_NATIVE_CALL is_user_connected(AMX *amx, cell *params) /* 1 param */
{
    int index = params[1];
    
    if (index < 1 || index > gpGlobals->maxClients)
        return 0;
    
    CPlayer* pPlayer = GET_PLAYER_POINTER_I(index);
    
    return (pPlayer->ingame ? 1 : 0);
}

// Here we see that is user alive is checking if the user is connect the same way the 
// is user connect is checking. So, if it was broken, both should be.
static cell AMX_NATIVE_CALL is_user_alive(AMX *amx, cell *params) /* 1 param */
{
    int index = params[1];
    
    if (index < 1 || index > gpGlobals->maxClients)
    {
        return 0;
    }
    
    CPlayer* pPlayer = GET_PLAYER_POINTER_I(index);

    if (g_bmod_tfc)
    {
        edict_t *e = pPlayer->pEdict;
        if (e->v.flags & FL_SPECTATOR || 
            (!e->v.team || !e->v.playerclass))
        {
            return 0;
        }
    }
    
    return ((pPlayer->ingame && pPlayer->IsAlive()) ? 1 : 0);
}