I’m assuming that your daughter is leaving Windsor before 09:00? In which case there is no saving using an 18-25 railcard.
In that case a return ticket to Slough is £4.20 a day, although this can be reduced to £2.84 per day (5 day week) if she buys a weekly season ticket from Windsor to Slough (£2.52 per day for a monthly season).
At Slough use contactless to tap the Yellow validator, and then at Ealing Broadway tap the Pink validator. When she gets to Chiswick Park she will have been charged £7.10 when she taps out.
For the return she should do the reverse making sure she taps the pink validator at Ealing Broadway, and the yellow validator at Slough. As this is outside of the peak period (06:30 to 09:30), this will be charged at £3.70.
So £4.20 + £7.10 + £3.70 equals £15.00