If someone would turn on internet connection using command
su -c "/sbin/ifup ppp1" would it put system at risk (how bad) ?
It makes no difference. You're just sending a command to bring up the interface; which user does this is irrelevant.
Assuming ppp1 is a Point to Point Protocol interface, this has a poor reputation amont security admins because it could be used to bypass any external firewall rules. Said differently if your system has a firewall that forbids a number of protocols or address ranges, but someone manages to open a PPP tunnel through it, the firewall will have no knowledge of what passes inside the tunnel, which somehow defeats its goal.
So my advice is that is a user sets up a PPP tunnel without an explicit authorization of the security administrator, this should be seen as an internal attack. Even if the user is not conscious of the problem, doing so actually lowers the global security level.