I think turning a 2nd level warlock with EWP: Whip and Weapon Finesse into a "front line fighter" is kind of a lost cause.
But, this is how I'd try to make the best of it.
"Front line fighters" have at least two of the following:
1) Resistance to taking physical damage (AC and HP, damage reduction)
2) Resistance to other attacks (good saves/resistances)
3) Ability to disable and/or injure enemies in melee (either through HP damage or tricks like tripping or disarming)
This fellow can't go the power attack route, as his str is too low. He also can't be a tripper (int isn't high enough for combat expertise). Archery/throwing things is viable, but that's not good for a front line fighter. So, #3, dealing damage/disabling opponents, isn't really possible without magic or sneak attack damage.
Thus, you could focus on getting your saves, AC, and HP as good as possible. Improved Toughness is a must. Save-boosting fights like Iron Will would help. Getting into the best possible armor is a must. I would try to get 5 ranks in tumble so I could fight defensively (maybe through a rogue level or bard level?) to boost my AC on the front line while still providing a flanking buddy for my rogues. I'd invest in potions of Mage Armor to get a nice +4 bonus to AC and I'd have a tower shield for those times when I didn't plan on casting.
OR....
The better option, IMHO, would be to just focus on being a good warlock. Take leadership at 6th level and attract a useful martial cohort. Until that time, hire some men-at-arms (a few SP/day can get you a 2nd or 3rd level warrior) or get a big mean riding dog as a guard dog. Tell one of the rogues to suck it up and take some fighter levels. They have a lot less to lose, as they aren't maintaining a spell casting progression. Alternatively, tell them to invest in Use Magic Device and have them buy some monster summoning scrolls.
Yes, you can't do any of those things until you are done with your current adventure, but that time will eventually come.