The following is a description of how Reflecting Jolteon #2 would fare against a Thick Bone Marowak:

If the switch is anticipated (and Ice Power is used), and Reflect is already up, then Marowak will get in one Earthquake, taking Jolteon down to a little less than half power, and Marowak will faint to the third Ice Power.  If the switch was not anticipated (but the Reflect is up and does not fade after the first Ice Power), if the Reflect was not up (but the switch was anticipated), or if the Reflect fades after the first Ice Power (but the switch was anticipated), then Jolteon will faint, leaving Marowak with little HP.  If both the switch was not anticipated and either the Reflect was not up or the Reflect fades after the first Ice Power, Jolteon will faint and Marowak will be left with greater than half HP.  All of the above scenarios assume the following:  Marowak was not already out on the turn Jolteon was sent out, Marowak was not sent out on the turn Jolteon was sent out, both Marowak and Jolteon were at full HP on the turn that Marowak was sent out, Jolteon has not undergone any stat changes, Marowak does not use any move other than Earthquake (ex. Restberry), no critical hits occur on either part, no moves miss, and something strange and inexplicable does not occur.

In many of the above situations in which Jolteon, the Reflect will still be partially usable by the next pokémon you send out (unless Jolteon was the last pokémon that hadn't been KOed).

Note:  If Marowak is already out on the turn during which Jolteon is sent out (and Marowak has undergone no stat changes previously) or if Marowak is sent out on the same turn as Jolteon, the battle will occur just as in a situation in which the switch to Marowak is not anticipated and the Reflect is not up (or fades after the first Ice Power).