Don't talk about 'battles' and 'fights' to beat cancer or mention any of those ghastly glossy cancer ads that ruin many a programme for those who are cursed with this godawful disease.
Tell her that, as always, you're there for her in good and bad times and you'll continue to celebrate or commiserate whatever is thrown her/your way..
Take food... strong cheese, sharp sweets like bitter lemon sherbets or similar that can override the temporary loss of taste buds that chemo can cause. Buy her a dark nail polish, black or very dark purple, to wear while she's having chemo as it can go some way to protect her finger and toenails from damage
Ask if there's anything you can do to make her daily life easier - do the washing up, cook a meal, stick the washing on/peg it out, look after the dc while she has a nap, drive/accompany her to appointments. or just make her a cuppa or whatever other beverage she prefers.
Above all, be yourself and no matter what, if any, change in her appearance, see her as the friend she's always been to you. This cancer shit can bring you closer together than you've ever been.