Generally your friend is right. Germanic and Celtic languages (along with Albanian, Armenian, Baltic, Greek, Indic, Iranian, Italic/Romance, and Slavic) all descend from the same ancestral language spoken by a group of people we call the Indo-Europeans.

These people had some practices/beliefs that they passed on to their descendants, including a "Sky Father" (Skt Dyaus Pita, Lat Iupiter, Grk Zeus Pater). A related root is contined by Old Norse Týr, Slavic div 'miracle', Skt deva 'benevolent god', and Avestan daeva 'malevolent god'. That last pair has an accompanying pair as well, Skt. asura 'malevolent god' and Av. ahura 'benevolent god' (these belong with Norse æsir, a group of gods). We're very fuzzy on the details of the practices and mythology of the Indo-Europeans, although Calvert Watkins wrote a book How to Kill a Dragon, where he reconstructs an Indo-European myth about dragon-slaying, on the basis of Irish and Indo-Iranian poetry about dragon-slayers.

The 'warrior culture' aspect, and the importance of horses are also clearly there, at least for the earliest groups. We see this especially in Indo-Iranian, which is replete with names referring to horses, chariots, and martial glory. This aspect of Indo-Iranian is even preserved in our records of the Mitanni, a Mesopotamian kingdom in the 15th-14th centuries BCE. They did not speak an Indo-European language, but their rulers had names that are clearly of Indo-Iranian extraction.

I am not a specialist, more of a hobby linguist/historian, but the best answer I can give is that one should not think of the Proto Indo-Europeans as existing in some kind of historical vacuum. The Celts, Germans, Gauls (which are a sub-set of the Celtic family, actually) are all linguistic and cultural descendants of the PIE family.

The timeline, of course, is far from concrete and deals more with tendencies and trends rather than succinct turning points, but the very simplified gist of it is that there existed a large group of steppe horsemen somewhere in Central Asia (you'll note that there have been many of these throughout history). Because they'd already mastered the horse (we know this because the word *ekwo- has been reconstructed from related terms in nearly all Indo European languages) they were able to spread with great rapidity. Minor aside: Any word that you read in a linguistic work with an asterisk (*) represents a reconstruction rather than an attribution. Because Proto Indo-European did not have a writing system, everything rendered as Proto Indo-European is a reconstruction. Anyone claiming otherwise is incorrect or speaking inaccurately. This may not have been a matter of military conquest, because languages can be transmitted through trade, and often without the people themselves. Think of how many peoples speak English as a secondary or foreign language, for a modern example of language transmission.

I'm sure someone better versed will be along shortly, but that's my rough understanding. There aren't too many affordable books on the subject, but I found J.P. Mallory's In Search of the Indo-Europeans fairly accessible. It's academic currency, however, I cannot speak to.

ETA: Rather than "*ekwos", Calvert Watkin's Dictionary of Indo-European Roots renders horse as "*ekwo-"

Are there any examples on how the PIE culture/civilization looked like?