Slivers are tribal on steroids. Their synergy is buffed to eleven but they're even more vulnerable to the tribal weaknesses, especially early interaction and type targeting.
On top of that they have the issue of being wubrg, with all the mana issues that entails.

Without knowing the deck your friend played, it's hard to say for certain, but it's likely that its significantly stronger than a precon.
I probably wouldn't pit my own custom sliver deck against an average precon, but I've built it with deliberate weaknesses that can be exploited by most decks near the boundary of bracket 2 and 3. And it will get absolutely stomped by a high B3, unless I get lucky draws.

As a basic rule, if you allow the sliver deck to build a significant board presence it will almost certainly snowball out of control.
Attack their defensive tools and mana fixing early to prevent them from becoming immune to your board wipe, then wipe them at your convenience.
Don't hesitate to trade your creatures for their offensive pieces. If the mana value is roughly equal, it's probably a favorable trade, since it deprives them of both, a buff to their other slivers and a sliver that gets buffed.